THE ATMOSPHERE 



77 



FIG. 75. Aneroid Barometer. 



of the atmosphere is below us. An ascent of one thou= 



sand feet causes a lowering of about one inch in the barom- 

 eter column. Barometers may 



thus be used to measure the 



height of the mountains or other 



elevations. For convenience in 



handling, a different type of 



barometer is much used for this 



work. It is called an aneroid 



barometer and consists of a small, 



flat, metal box from which the 



air has been exhausted (Figure 



75). A variation in the pressure 



of the atmosphere causes a slight 



change in the shape of the little 



box. This change is read on a dial as atmospheric pres- 

 ^_ ^ sure, in terms of a column of mercury. 

 One half of the atmosphere is within a 

 distance of less than four miles of the 

 earth's surface, and the barometer read- 

 ing at such a height is about fifteen 

 inches. 



Experiment 17. Into a glass tube three 

 fourths of an inch in diameter fit a plunger 

 (Figure 76). Place the bottom of the outer 

 tube in water and raise the plunger. Why does 

 the water rise in the tube ? In case a common 

 glass pump is available -it may be used for this 

 experiment. 



Pumps. The common pump (Figure 

 77) removes the air from the pump stock 



FiG.76.-Diagram 

 Showing Action of 



piston. 



of the spout. The weight of the water 



